1. Field of the Invention
This invention relates generally to a method and apparatus for photographing a radiation image, and more particularly to a radiation-image photographing method and apparatus that obtains the radiation-image information of a subject by detecting radiation transmitted through the subject with a solid radiation detector.
2. Description of the Related Art
Today, in the field of radiation photography with the object of medical analysis and the like, a wide variety of radiation-image photographing apparatuses have been proposed and put to practical use (see U.S. Pat. Nos. 5,514,873 and 5,354,982, Japanese Unexamined Patent Publication No. 8(1996)-299316, etc.). In the radiation-image photographing apparatus, radiation transmitted through a subject is detected by a solid radiation detector (which has a semiconductor as a major part and will also be hereinafter referred to simply as a detector) in order to obtain an image signal representative of a radiation image related to the subject.
Also, various types of solid radiation detectors to be used in the radiation-image photographing apparatus have been proposed. For instance, with respect to the charge-generating process of converting radiation to an electric charge, there is a solid radiation detector of light conversion type (see Japanese Unexamined Patent Publication Nos. 59(1984)-211263 and 2(1990)-164067, PCT International Publication No. WO 92/06501, SPIE Vol. 1443, xe2x80x9cMedical Image V,xe2x80x9d Image Physics (1991), pp. 108-119, etc.). In this type of detector, a fluorescent substance is irradiated with radiation and emits fluorescent light. The fluorescent light is detected by a photoelectric conversion element (which is a solid detector). The resultant signal charge is temporarily stored in the electricity storage portion of the photoelectric conversion element, and the stored charge is converted to an image signal (electric signal) and is output. There is also a solid radiation detector of direct conversion type (see xe2x80x9cMATERIAL PARAMETERS IN THICK HYDROGENATED AMORPHOUS SILICON RADIATION DETECTOR,xe2x80x9d Lawrence Berkeley Laboratory and University of California, Berkeley, Calif. 94720 Xerox Parc, Palo Alto, Calif. 94304, xe2x80x9cMetal/Amorphous Silicon Multilayer Radiation Detectors,xe2x80x9d IEE TRANSACTION ON NUCLEAR SCIENCE, Vol. 36, NO. 2 April 1989, Japanese Unexamined Patent Publication No. 1 (1989)-216290, etc.). In this type of detector, a signal charge, produced in a radiation conductor irradiated with radiation, is collected by a charge collecting electrode and is temporarily stored in an electricity storage portion. The stored charge is converted to an electric signal and is output.
In addition, with respect to the charge-reading process of reading out a stored electric charge to the outside, there is a thin-film transistor (TFT) reading type which reads out an electric charge by scanning and driving a TFT connected to the electricity storage portion, an optical reading type which emits reading light (reading electromagnetic wave) to a detector and reads out an electric charge, and so on.
Furthermore, the applicant of this application has proposed an improved direct conversion type of solid radiation detector in Japanese Unexamined Patent Publication Nos. 10(1998)-232824 and 10(1998)-271374. The improved solid radiation detector is of a direct conversion and optical reading type and is formed by stacking (1) a recording photoconductive layer which exhibits photoconductivity by receiving the emission of recording radiation directly or indirectly, (2) a charge transfer layer which operates as substantially an electric conductor with respect to an electric charge of the same polarity as a latent image charge and also operates as substantially an insulator with respect to a transfer charge of the opposite polarity from the latent image charge, and (3) a reading photoconductive layer which exhibits photoconductivity by receiving the emission of a reading electromagnetic wave, in the recited order. In this improved type of detector, a signal charge (latent image charge) carrying image information is stored in the interface (electricity storage portion) between the recording photoconductive layer and the charge transfer layer. Also, electrodes (first and second conductive layers) are stacked on both sides of these 3 layers. The recording photoconductive layer, the charge transfer layer, and the reading photoconductive layer form the major part of the solid radiation detector in this type.
In photographing a radiation image, incidentally, a predetermined voltage is applied across the above-mentioned detector (which is, for example, a solid detector such as a photoelectric conversion element) to emit radiation, whereby an electric charge representing radiation-image information is stored in the electricity storage portion. On the other hand, after photographing, the application of voltage is stopped, or a standby voltage less than the above-mentioned predetermined voltage is applied. In this way, the electric charge stored in the electricity storage portion is held and the occurrence of dark current, which will be caused by continuing to apply the aforementioned predetermined voltage, is prevented.
In the case of using the optical reading type detector, there are cases where preexposure light is emitted to the reading electrode side to remove unnecessary electric charges stored in the electricity storage portion before the emission of recording light (which unnecessary electric charges include (1) a residual electric charge left without being read out after electric charges in the electricity storage portion have been read out, (2) dark current which occurs as voltage is applied to the device, and so on), as described in the above-mentioned Japanese Unexamined Patent Publication No. 10(1998)-271374.
Some of the direct conversion type detectors can perform recording immediately after dc voltage has been applied between both ends of the detector to form a predetermined electric field within the detector, as described in the above-mentioned Japanese Unexamined Patent Publication No. 10(1998)-271374, and others need to perform real recording after the detector has been pre-charged by primary exposure before recording, as described in xe2x80x9c23027 Method and device for recording and transducing an electromagnetic energy pattern (Research Disclosure, June 1983)xe2x80x9d, for example.
Furthermore, for instance, as described in Japanese Unexamined Patent Publication Nos. 61(1986)-244176 and 9(1997)-206293, some of the light conversion type detectors need to supply a predetermined electric charge to the electricity storage portion in advance to fully charge the electricity storage portion prior to photographing.
However, for example, the above-mentioned U.S. Pat. No. 5,514,873 discloses only that with respect to a housing having a solid radiation detector (radiation converter), memory, an energy source, and a radio transmitter, the supply of energy to the converter is performed from the outside by radio. There is no description of the application of voltage to the detector and the timing of emitting radiation. Therefore, what is disclosed in this reference alone can neither necessarily store an electric charge in the electricity storage portion with reliability nor prevent dark current.
Also, the above-mentioned U.S. Pat. No. 5,354,982 discloses that the voltage applied to the device is lowered after the generation of a latent image in order to hold the latent image, but has the disadvantage that if the application of voltage is continued, a signal-to-noise (S/N) ratio will be reduced by the storage of dark current.
Furthermore, the above-mentioned Japanese Unexamined Patent Publication No. 8(1996)-271374 discloses that the X-rays transmitted through the solid radiation detector are detected with an X-ray sensor provided behind the detector and, based on the result of detection, the amount of the transmitted X-rays is controlled. However, there is no description as to how the timing of emitting the X-rays is controlled. Therefore, what is disclosed in this reference alone can neither always store an electric charge in the electricity storage portion reliably nor prevent dark current.
In the conventional apparatuses, the application of voltage to the detector, the emission of radiation, the emission of preexposure light, primary exposure, and the charging start and stop of the electricity storage portion are not always operated at proper timings. For this reason, dark current will increase and an unnecessary electric charge will not be removed, and consequently, there is a problem that the noise in an image signal output from the detector will increase and therefore the S/N ratio in the reproduced image will be reduced.
The present invention has been made in view of the aforementioned problems found in the prior art. Accordingly, the primary object of the present invention is to provide a radiation-image photographing method and apparatus which is capable of performing the application of voltage to the solid radiation detector, the emission of recording radiation or preexposure light, etc. at appropriate timings.
The appropriate timings of the start and stop of volt.age application to the solid radiation detector, the start and stop of radiation emission, and the start and stop of emission of an electromagnetic wave for preexposure are as follows. To store an electric charge produced by the emission of recording radiation in the electricity storage portion of the solid detecting element, a predetermined voltage has to be applied across the detector, so it is desirable to start the emission of radiation immediately after the application of voltage to the detector or after the detector has become stable since the application of voltage. That is, it is desirable that the start of voltage application and the start of radiation emission be performed at almost the same time.
From the viewpoint of the prevention of the occurrence of dark current after photographing, it is desirable that the voltage across the detector be stopped or switched to standby voltage in a short time period after the stop of radiation emission. That is, it is desirable that the stop of voltage application and the stop of radiation emission be performed at almost the same time.
On the other hand, in performing preexposure in the case of using an optical reading type detector, it is desirable, from the viewpoint of the removal of an unnecessary electric charge in addition to the above-mentioned timings, that preexposure be started near the start of voltage application to the detector. That is, it is desirable that the application of voltage to the detector and the start of preexposure light emission be performed at almost the same time.
Similarly, from the viewpoint of the removal of an unnecessary electric charge, preexposure is continued up to just before the start of radiation emission. In other words, it is desirable to emit radiation within a short time period after the stop of preexposure. That is, it is desirable that the stop of preexposure light emission and the start of radiation emission be performed at almost the same time.
Furthermore, in the case of using a direct conversion type detector that requires primary exposure, it is desirable to start primary exposure near the application of voltage to the detector and start the emission of radiation after the completion of pre-charging (full charging). That is, it is desirable that at least the stop of primary exposure and the start of radiation emission be performed at almost the same time.
The present invention renders it possible to perform the start of voltage application and the start of radiation emission and the like automatically at almost the same time. More specifically, a first radiation-image photographing method according to the present invention is a radiation-image photographing method for obtaining radiation-image information of a subject, the method comprising the steps of:
detecting radiation transmitted through the subject by a solid radiation detector having a plurality of solid detecting elements; and
performing (1) start of voltage application to the solid radiation detector, which is required for storing an electric charge representative of the radiation-image information in the solid detecting elements, and start of the radiation emission and/or (2) stop of the voltage application and stop of the radiation emission at almost the same time.
A second radiation-image photographing method according to the present invention is a radiation-image photographing method for obtaining radiation-image information of a subject, the method comprising the steps of:
detecting radiation transmitted through the subject by a solid radiation detector having a plurality of solid detecting elements; and
performing (1) start of voltage application to the solid radiation detector, which is required for storing an electric charge representative of the radiation-image information in the solid detecting elements, and start of an electromagnetic wave emission for preexposure prior to the detection and/or (2) stop of the electromagnetic wave emission and stop of the radiation emission at almost the same time.
A first radiation-image photographing apparatus according to the present invention is a radiation-image photographing apparatus for obtaining radiation-image information of a subject, the apparatus comprising:
a radiation source for emitting radiation;
a solid radiation detector for detecting the radiation transmitted through the subject, the detector having a plurality of solid detecting elements which are driven by voltage-applying means; and
control means for controlling the radiation source and the voltage-applying means so that (1) start of voltage application to the solid radiation detector, which is required for storing an electric charge representative of the radiation-image information in the solid detecting elements, and start of the radiation emission and/or (2) stop of the voltage application and stop of the radiation emission are performed at almost the same time.
In a preferred form of the first radiation-image photographing apparatus, the apparatus further comprises detection means for detecting the presence of the emission of the radiation, and the control means starts the application of the voltage when the detection means detects that the emission of the radiation is present, and/or stops the application of the voltage when the detection means detects no emission of the radiation after the radiation has been emitted.
As the detection means for detecting the presence of the emission of recording radiation, any type of detection means may be used, as long as it is able to detect the presence of radiation, transmitted through a subject and incident on the detector, or detect what is in a corresponding relationship with the radiation. The detection means is not limited to detecting radiation itself transmitted through a subject, but may be any detector which detects radiation emitted from the radiation source corresponding to the radiation incident on the detector or may be any detector that detects light issuing from the detector irradiated with radiation. For instance, a photo timer, a photodiode and the like are preferred. Note that because current flows in the detector when recording radiation is emitted to the detector, the detector itself can function as the detection means for detecting the presence of the emission of radiation.
A second radiation-image photographing apparatus according to the present invention is a radiation-image photographing apparatus for obtaining radiation-image information of a subject, the apparatus comprising:
a radiation source for emitting radiation;
a solid radiation detector for detecting the radiation transmitted through the subject, the detector having a plurality of solid detecting elements which are driven by voltage-applying means; and
preexposure means for emitting an electromagnetic wave for preexposure which is performed prior to the detection;
wherein the voltage-applying means and the preexposure means are controlled so that start of voltage application to the solid radiation detector, which is required for storing an electric charge representative of the radiation-image information in the solid detecting elements, and start of the electromagnetic wave emission are performed at almost the same time and/or the radiation source and the preexposure means are controlled so that stop of the electromagnetic wave emission and start of the radiation emission are performed at almost the same time.
In a preferred form of the second radiation-image photographing apparatus, the apparatus further comprises detection means for detecting the presence of the emission of the electromagnetic wave, and the control means starts the application of the voltage when the detection means detects that the emission of the electromagnetic wave is present, and/or starts the emission of the radiation when the detection means detects no emission of the electromagnetic after the electromagnetic wave has been emitted.
The xe2x80x9cemission of recording radiationxe2x80x9d in the above-mentioned methods and apparatuses means that the radiation representative of the radiation-image information on a subject, emitted from the radiation source and transmitted through the subject, is emitted to the detector.
The xe2x80x9capplication of voltage to the solid radiation detector which is required for storing an electric charge representative of radiation-image information in the solid detecting elementsxe2x80x9d means that necessary voltage is applied to the detector to store an electric charge representative of radiation-image information, generated in the charge generating portions of the solid detecting elements of the detector by emitting the recording radiation to the detector, in the electricity storage portions of the solid detecting elements. The application of voltage is not limited to the application of a desired voltage to a solid detecting element such as a photoelectric conversion element but includes the application of a desired voltage to other portions of the detector necessary for the aforementioned storage.
The xe2x80x9cemission of an electromagnetic wave for preexposurexe2x80x9d means emitting an electromagnetic wave other than recording light and reading light to the recording surface side or reading surface side of the detector prior to the emission of the recording light to the detector
The phrase xe2x80x9calmost the same timexe2x80x9d in the above-mentioned methods and apparatuses means the relation of time that the start and stop of voltage application to the solid radiation detector, the start and stop of radiation emission, and the start and stop of electromagnetic wave emission for preexposure are performed at the aforementioned respective appropriate timings. The phrase xe2x80x9calmost the same timexe2x80x9d is sufficient if it is the above-mentioned appropriate timing, and does not always need to be at exactly the same time. Note that the detector, which is used in the method and apparatus of the present invention, may be of any type and can use detectors of various types, such as the above-mentioned light conversion type, direct conversion type, TFT reading type, optical reading type, improved direct conversion type which is a combination of them, and the like. For example, the appropriate timings of the start of voltage application and the start of radiation emission vary depending on detectors to be used. Therefore, it is a matter of course that the application of voltage and the like need to be performed at appropriate timings in accordance with a detector to be used.
Here, the timing control (time management) method for performing the start of voltage application and the start of radiation emission and the like at almost the same time may be any method, if it can automatically perform start and stop operations so that the start of voltage application and the start of radiation emission and the like are performed at the above-mentioned appropriate timings. For instance, based on a photographing start instruction from an operator, a timer can be started so that the start and stop of voltage application, emission of radiation and the like are performed at respective set times.
According to the first radiation-image photographing method and apparatus, respective timings are caused to automatically coincide with each other so that the start of voltage application to the solid radiation detector and the start of recording radiation emission are performed at almost the same time. Therefore, the electric charge produced by the emission of recording radiation can be stored in the electricity storage portion with reliability and the storage of an unnecessary electric charge due to dark current before emission can be prevented.
In addition, if respective timings are caused to automatically coincide with each other so that the stop of voltage application and the stop of recording radiation emission are performed at almost the same time, an unnecessary electric charge can be prevented from being stored after photographing. With this control, the S/N ratio during read operation is enhanced.
According to the second radiation-image photographing method and apparatus, respective timings are caused to automatically coincide with each other so that the start of voltage application to the solid radiation detector and the start of recording radiation emission are performed at almost the same time, or so that the stop of voltage application and the stop of radiation emission are performed at almost the same time. Therefore, an electric charge corresponding to the ray amount of the recording radiation can be stored after an unnecessary electric charge remaining within the detector has been removed.
According to the methods and apparatuses of the present invention, as described above, the possibility of giving rise to the problems of the increase of dark current and an unremoved unnecessary electric charge is reduced, so that a S/N ratio in an image, reproduced based on an image signal output from the detector, can be improved compared with manual operation.
The conventional apparatus, as in the apparatuses disclosed in the above-mentioned U.S. Pat. Nos. 5,514,873 and 5,354,982, Japanese Unexamined Patent Publication No. 8(1996)-299316 and the like, does not necessarily perform operations at appropriate timing from the viewpoint of the occurrence prevention of dark current, removal of an unnecessary electric charge, and full charging, whereas the present invention can reduce the possibility of increasing noise in an image signal and can improve a S/N ratio in an reproduced image, because respective timings are caused to automatically coincide with each other so that the application stop of voltage and the emission stop of recording radiation are performed at nearly the same time.